The present invention relates to a method and system for improving engine braking. In particular, the present invention relates to methods and systems using variable valve operation to improve engine braking performance.
Valve actuation in an internal combustion engine is required in order for the engine to produce positive power. During positive power operation of an engine, one or more intake valves may be opened to allow air and fuel into a cylinder for combustion. This intake event is routinely carried out while the piston in the cylinder travels from a near top dead center (TDC) position to a near bottom dead center (BDC) position. After the intake stroke, the intake valve(s) are closed and the air/fuel charge in the cylinder is compressed as the piston travels back from the BDC position to a TDC position during a compression stroke. The compressed mixture is combusted around TDC, which drives the piston back toward a BDC position during what is known as an expansion stroke. Following the expansion stroke, one or more exhaust valves that communicate with the cylinder may be opened to allow the combustion gas to escape therefrom. The foregoing intake and exhaust valve events are commonly referred to as the main intake and main exhaust events, respectively.
During engine braking, the exhaust valves may be selectively opened to convert, at least temporarily, a power producing internal combustion engine into a power absorbing air compressor. As a piston travels upward during its compression stroke, the gases that are trapped in the cylinder are compressed. The compressed gases oppose the upward motion of the piston. During engine braking operation, as the piston nears TDC, at least one exhaust valve is opened to release the compressed gases to atmosphere, preventing the energy stored in the compressed gases from being returned to the engine on the subsequent expansion down-stroke. In doing so, the engine develops retarding power to help slow the vehicle down.
The operation of a compression-release type engine brake, as described in the preceding paragraph, has long been known. One of the earliest descriptions of a system used for compression-release braking is provided in Cummins, U.S. Pat. No. 3,220,392. The system described in the Cummins ""392 patent derives the motion to open a pair of exhaust valves for a compression-release event from an existing intake, exhaust, or injector pushrod or rocker arm. The compression-release motion is conveyed from a pushrod or rocker arm to a bridge joining two exhaust valves by a selectively expandable hydraulic linkage. This hydraulic linkage is expanded to convey the compression-release motion during engine braking operation, and contracted to absorb such motion during positive power operation. The contraction of the hydraulic linkage during positive power operation causes the compression-release motion to be xe2x80x9clostxe2x80x9d during positive power, and accordingly, such systems are commonly referred to as xe2x80x9clost motionxe2x80x9d valve actuation systems.
As shown in the Cummins ""392 patent, many contemporary engines are multi-valve engines that employ, for example, four valves per cylinder, i.e., two intake valves and two exhaust valves, in order to improve overall performance. The conventional multi-valve actuation system typically opens both intake or both exhaust valves for a particular cylinder simultaneously. For example, in various embodiments described in the Cummins ""392 patent, both of the exhaust valves for a given cylinder are actuated (opened and closed) simultaneously for a compression-release event. Because the two exhaust valves are actuated in response to motion imparted by a single source, both exhaust valves are provided with substantially the same lift and duration, in addition to being provided with substantially identical timing.
Over the years there have been various improvements to the systems and methods described in the Cummins ""392 patent. One such improvement is described in Jakuba et al., U.S. Pat. No. 4,473,047. Like the system described in the Cummins patent, the Jakuba patent describes the use of a lost motion system in conjunction with an engine having two exhaust valves per cylinder. However, unlike the system described in the Cummins patent, the system described in the Jakuba patent conveys the compression-release motion to only one of the two exhaust valves associated with each engine cylinder. The inventors of the Jakuba system stated that they, xe2x80x9cdiscovered that by opening only one of the exhaust valves during engine braking a surprising increase in retarding horsepower can be achieved. The increase in retarding horsepower is accompanied by a decrease in the observed operating pressure in the hydraulic system and is related to a decrease in the overall load in parts of the braking system.xe2x80x9d
For a system designed for two-valve braking with one rocker arm, the braking load is basically cut into half by opening only one valve if the same peak cylinder pressure is maintained before compression-release blow-down. Therefore, the system should be able to sustain much higher cylinder pressure by a later opening of one valve to achieve higher retarding power and lower overall braking load at the same time. As described in more detail below, the Applicant has determined that if two individual rocker arms are used to open the two valves independently, then opening two valves is better than opening one due to faster compression-release blow-down from the same high peak cylinder pressure since braking load is not an issue for two valve braking with two rocker arms.
Other improvements over the system described in the Cummins patent have involved hardware, which falls into two broad categories: lost motion systems, and common rail systems. Several advancements in lost motion systems have been made to accommodate the modern prevalence of overhead cam engines. For example, recent lost motion system advancements have involved the placement of the hydraulic linkage in expandable tappets between a cam and a rocker arm or the engine valve itself, such as shown in Vorih et al., U.S. Pat. No. 5,829,397, which is hereby incorporated by reference. Lost motion components have also been integrated into rocker arms, such as is shown in Cartledge, U.S. Pat. No. 3,809,033, and Hu, U.S. Pat. No. 5,680,841, which are hereby incorporated by reference. Still other lost motion advancements, such as those shown in Vorih, U.S. Pat. No. 6,085,705 and which is hereby incorporated by reference, have been made to enable variable valve actuation (WA), which provides for the modification of individual valve actuation events on an engine cycle-by-cycle basis.
In the lost motion systems described above, the engine valves are typically driven by fixed profile cams, more specifically, by one or more fixed lobes on each of the cams. The use of fixed profile cams makes it difficult to adjust the timing and/or magnitude of the engine valve lift needed to optimize engine performance for various engine operating conditions, such as different engine speeds during engine braking. Rapid adjustment of valve timing in a system utilizing fixed profile cams is only now becoming viable using WA systems such as the one described in the Vorih ""705 patent.
In common rail valve actuation systems, a source of high pressure hydraulic fluid is selectively applied to a piston to actuate the one or more exhaust valves for the compression-release events. Examples of such systems are shown in Meistrick et al., U.S. Pat. Nos. 5,787,859, 5,809,964, and 6,082,328, which are hereby incorporated by reference.
Common rail systems may provide virtually limitless adjustment to valve timing because the source of high pressure hydraulic fluid is constantly available for valve actuation. Accordingly, given sophisticated and high speed control over the application of this hydraulic pressure, a common rail system should be able to deliver valve actuation on demand, as well as provide some control over lift and duration. To date, however, such sophisticated control, particularly in the seating of engine valves has not been effectively realized. Two problems in particular that tend to discourage the use of common rail actuation systems are the expense of the components required to exercise the level of control called for, and the susceptibility of the system to complete failure in the event of a loss in hydraulic pressure. Until these problems are solved, it is likely that lost motion systems will continue to be the predominate type of system used to carry out engine braking.
The ideal compression-release braking cycle should have both the maximum (peak) and minimum cylinder pressures occur at the compression TDC, which means that the braking valve(s) would not be opened until TDC and then the compression-release blow-down event would happen instantaneously. Therefore, a combination of late valve opening toward TDC and then a fast compression-release blow-down after the TDC maximizes engine braking power
Compression-release (or valve actuation) timing is controlled by braking load. The closer the piston is to TDC, the greater the pressure in the cylinder, and accordingly, the greater the load placed on the elements that must carry out the valve opening event. Increased braking loads result in increased loads on both the structural components and the hydraulic fluid used to carry out a compression-release event. With increasing load, the structural components may be deformed and hydraulic compliance may be increased, which may affect the timing and degree of exhaust valve actuation for a compression-release event. Small losses due to structural deformation and hydraulic compliance could potentially result in loss of the entire compression-release event because of the relatively small magnitude of the event to begin with. Thus, component strength and hydraulic compliance limit the piston position at which a system is capable of initiating a compression-release event relative to TDC.
The compression-release (or blow down) speed is controlled by valve opening area that could be increased by increasing the number of exhaust valves for the braking event. Therefore, opening two exhaust valves would achieve higher braking power than opening only one valve for compression-release of braking gases from the same peak cylinder pressure.
It is also known that fixed timing compression-release valve actuation systems provide optimal engine braking power for only one engine speed. Compression-release actuation for high engine speeds may be constrained by valve-train loading limits that necessitate advancing the time before TDC at which the exhaust valve(s) are opened. The advancement of the compression-release event for high engine speeds, however, provides reduced braking power at low engine speeds.
While a WA system or a common rail system could provide optimal engine braking power for a range of engine speeds, such systems tend to be complex and costly. Accordingly, there is a need for a method of valve actuation that provides improved engine braking power at a plurality of engine speeds without necessitating the use of a complicated WA system. There is also a need for a method of valve actuation that provides improved engine braking power without subjecting the system used for such braking to undesirably high loads.
To date, the Applicants are unaware of any system that actively determines the number of exhaust valves that should be actuated to optimize braking power. The Applicants have further determined that such a system could be used to optimize braking power over a range of engine speeds, as well as reduce the load placed on the engine braking elements at some engine speeds. Thus, there is a need for a system and method that is capable of determining whether two or one exhaust valves should be opened for an optimal engine braking event. Furthermore, there is a need for a system and method that can change between actuating one or two exhaust valves for engine braking based on the determination of which will provide optimal braking power and/or optimal engine braking element loading.
As explained above, normally the advancement of the opening time of the exhaust valves for a compression-release event will decrease braking power because there is less pressure in the cylinder to release. To some extent, however, this loss of power must be tolerated because of the increased load experienced by the system as the opening event is moved closer to TDC. Thus, there is a need for a method of engine braking that takes advantage of the lower loading resulting from initiating the compression-release event at an earlier time in the cycle, but avoids the drastic loss of braking power that usually accompanies the earlier initiation of the compression-release event.
It is known that staggering the opening times of intake and exhaust valves may be used to improve fuel economy, reduce exhaust emissions, and increase positive power. Such a system is described in King, U.S. Pat. No. 5,003,939. While such a system has been used to improve positive power performance, Applicants are unaware of any discussion of the staggering of the opening times of exhaust valves to optimize compression-release engine braking. In this regard, the Applicants have determined that the loading of the elements used to open two exhaust valves for a compression-release event may be reduced without a substantial loss in braking power by staggering the times at which each of the two exhaust valves are opened relative to TDC. Thus, there is a need for a system and method that is capable of staggering the opening of or sequentially opening two exhaust valves for a compression-release event.
Therefore, it is an object of the present invention to provide improved engine braking using variable valve operation.
It is another object of the present invention to provide a system and method for improving engine braking by switching between multiple valve actuation and single valve actuation.
It is another object of the present invention to provide a system and method for improving engine braking by using sequential valve actuation.
It is another object of the present invention to provide a system and method for improving engine braking by varying valve lift.
It is still another object of the present invention to provide a system and method for reducing valve train loading during engine braking.
It is yet another object of the present invention to provide a system and method for reducing valve train compliance during engine braking.
It is another object of the present invention to provide a system and method for optimum operation of the engine brake over a range of engine speeds by controlling the number of exhaust valves that open, and the timing and the lift of each valve.
It is still another object of the present invention to provide a system and method for reducing the number, weight and size of various engine components required for engine braking.
It is still another object of the present invention to provide improved engine performance during firing (positive power) cycles by controlling the numbers of valves which open, the timing and the lift of each valve.
It is still another object of the present invention to improve engine start and warm up by controlling the numbers of valves which open, the timing and lift of each valve and more specifically by operating some cylinders in a positive power mode and some cylinders in a braking mode simultaneously.
Additional objects and advantages of the invention are set forth, in part, in the description that follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.
The present invention is directed to a method and system for using variable valve operation to improve engine braking performance. In a preferred embodiment, the present invention is a method of optimizing engine braking power for multiple engine speeds in a multi-valve internal combustion engine. The method comprises the steps of selecting an engine speed as a cross-over point between one-valve engine braking and multi-valve engine braking; measuring an engine parameter to determine the current engine speed; determining whether the current engine speed is above, equal to, or below the cross-over point engine speed; and modifying the operation of at least one engine valve responsive to the determination of whether the current engine speed is above, equal to, or below the cross-over point engine speed.
The step of modifying the operation of at least one engine valve may comprise the step of modifying the number of engine valves actuated, the step of modifying the timing of the operation of at least one engine valve, and/or the step of modifying the lift of at least one engine valve. The engine valve may include an intake and/or an exhaust valve.
In another preferred embodiment, the present invention is a valve actuation system for actuating at least one engine valve to produce an engine valve event in a multi-valve internal combustion engine. The valve actuation system may comprise a housing, having a fluid linkage formed therein; means for selectively displacing hydraulic fluid located in the fluid linkage; means for controlling the displacement of the hydraulic fluid in the fluid linkage to modify the operation of the at least one engine valve responsive to a determination of the current engine speed; and means for actuating the at least one engine valve to produce the engine valve event, wherein the actuation means is slidably received in the housing and operatively connected to the displacement means through the fluid linkage.
The displacement control means may modify the number of engine valves actuated, the timing of the engine valves actuated, and/or the lift of the engine valves actuated. The engine valve event may be an intake valve event, a compression release engine braking event, a bleeder braking event, and/or an EGR event.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated herein by reference, and which constitute a part of this specification, illustrate certain embodiments of the invention and together with the detailed description serve to explain the principles of the present invention.